High-frequency switches have the function to pass or block high-frequency signals. In the passing case, they should be characterized by an ohmic resistor, which is as small as possible, and in the blocking case, by a constant capacitance, which is as small as possible. They can be realized by different types of switching elements. In integrated circuit technology, high-frequency switches are frequently realized by using a silicon substrate. Thereby, generally, the two types of bipolar or MOS transistor can be formed. In the case of a silicon bipolar transistor, the passing case is easy to realize, when the same is operated with sufficiently high emitter base current in the triode region of the characteristic curve far below the collector current saturation. With correct transistor dimensioning, this control current can remain smaller than 1 mA. The blocking case causes more difficulties, since here a base or bias voltage, which is as high as the high-frequency amplitude to be blocked, is required in the off-state. In other words, it can be said that a base emitter bias voltage has to be applied in reverse direction with an amount of at least the amplitude of the high-frequency signal, so that the collector base diode is not polarized (controlled) in flow direction. Thereby, the available bias voltage limits the switchable power. This causes the problem, for example in battery-operated mobile radio systems, that a required bias voltage of about 20 Volt is substantially higher than an operating voltage of the mobile radio system of, for example, 2.8 Volt.
High-frequency switches for higher powers are realized outside the RF-ICs (RC-IC=radio frequency integrated circuit) in the form of pin diodes or GaAs transistor switches, which can be operated with low or without bias voltage, respectively. However, disadvantages are the higher costs incurred by the additional devices. A further disadvantage of pin diodes as high-frequency switches is that pin diodes require a partly high switching time, which makes their usage problematic in high or ultrahigh frequency technique, respectively.
EP 1542287A1 discloses a high-frequency switching transistor 100, as is illustrated in FIG. 3. The high-frequency switching transistor 100 comprises a collector region 104, which has a first conductivity type, a first barrier region 108 bordering on the collector region 104, which has a second conductivity type, which differs from the first conductivity type, and a semiconductor region 114 bordering on the first barrier region 108, which has a dopant concentration, which is lower than the dopant concentration of the first barrier region 108. Further, the high-frequency switching transistor 100 has a second barrier region 120 bordering on the semiconductor region 114, which has the first conductivity type, as well as base region 112 bordering on the second barrier region 120, which has the second conductivity type. Additionally, the high-frequency switching transistor 100 comprises a third barrier region 128 bordering on the semiconductor region 114, which has the second conductivity type and a higher dopant concentration than the semiconductor region 114. Further, the high-frequency switching transistor 100 has an emitter region 130 bordering on the third barrier region 128, which has the first conductivity type.
By such a high-frequency switching transistor, it is possible to switch high-frequency signals of high amplitude with low distortion. The distortion can thereby be described by the generation of harmonics at sinusoidal control. The even harmonics (H2, H4, etc.), particularly the vibration H2 are thereby caused mainly by asymmetries in the vertical doping profile of the high-frequency switching transistor 100. The NPIPN doping profile of the high-frequency switching transistor illustrated in EP 1542287A1 cannot be realized fully symmetrically in production, so that the generation of the higher H2 vibration can become a problem.
Such a problem of generating higher harmonics can be compensated by two different approaches.
First, a series connection of two switches in opposite polarity can be used. However, this increases the switching effort significantly, because both the transistors and the bias coupling-in switching elements (e.g. resistors or coils) have to be doubled.
As a second method of reducing H2 wave generation, a longer I zone (i.e. a thick semiconductor region 114) can be used. Hereby, the space requirements are also increased, because the emitter area has to be increased in proportion to the I zone length.